Transmission control



April 18, 1933 D. MITCHELL TRANSMISSION CONTROL Filed Sept. 18, 1931INVENTOR .D. jyllfchell BY c ATTORNEY Patented Apr. 18, 1933 UNITEDSTATES PATENT OFFICE- i I DOREN MITCHELL, F MARTINSVILLE, NEW JERSEY,ASSIGNOR TO AMERICAN TELE- PHONE AND TELEG-EAPH COLIPANY, A CORPORATIONOF NEW YORK "TRANSMISSION I con'rnon Application filed September 18,1931. Serial No. 563,646.

This invention relates to systems for the transmission of energy andmore particularly to telephone systems, over which are transmitted theelectrical waves representing speech, With greater particularity theinvention' relates to telephonecircuits-especially four-wire circuitsinwhich transmission is controlled by switching-devices responsive toenergy in the transmission paths.

In a telephone circuit it is often desirable to maintain a normal ornon-operating conclition of disability in order to preventfising ing. Itthen becomes necessary, ofcourse, to cause the removal of thisdisability when speech energy is transmitted. In the case of a four-wirecircuit, in which the path normally disabled is associated with a pathfor transmission in the opposite direction (usually through a hybridcoil connection to an extending two-wire circuit), unbalance conditionsmay and usually do result in' the reflection of energy transmitted overthe latter path into the former. Again, the normally operative one-waypath "usually has associated with it wave responsive devices forcontrolling the transmission over the four-wire circuit. If thisnormally operative path is adapted for transmission from a link'of thesystem which is noisy or unstab le'a link whichm-ay be termed generically' mutablesuch as a radio link, which may be noisy due to static,the interfering energy may cause undesired operation of the transmissioncontrol apparatus.

Accordingly, it becomes desirable to effect adjustments of the volumesin the trans mission paths and adwstme'nts of the sens1-' tivities ofthe wave responslve'apparatus for controlling transmission, with a viewto as suring satisfactory operation of the wave responsive apparatus andat the same time preventing false or undesired operation of suchapparatus. In this connection various factors must be taken intoconsideration; For

instance, when energy comes in over the receiving one-way path of afour-wire circuit, the return loss which is determined by the degreeofunbalance between the connected two-wire line and the artificialnetworkaffects the amount of echo transmission occurring in thetransmitting path of-the fourw'ire circuit. Again, the ability of alocal talker to operate the transmission control apparatus responsive toenergy'in the transmitting path determines the volume control which isdesirable in the transmitting path. If itbecoines necessary to makeaconsiderable increase in gain in the transmitting path and the returnloss is low, it will be understood that it becomes desirable to lowerthe volume in the receiving path to prevent falseoperation of thetransmitting control apparatus by echoes of incoming energy which wastoo weak to operate the receiving controlapparatus. Further, it isalwaysdesirable to maintain the Volume in the receiving path at the highestpoint consistent with the prevention of the above-rnentioned falseoperation; p The principal obj ect of the present invention is toprovide for the lowering of the volume inlthereceiving path of afour-wire circuit when the loss in the ,echo path becomes low enough tothreaten false operation of the switches controlled by the transmittingpa'th but to retain the normal Volume in the receiving path until thislowered loss renders the change necessary. 'In'general, the applicant,in accomplishing the above stated object, utilizes a variable impedancebridged across each of the paths of the four-wire circuits; increasesthe transmitting path impedance when this is' necessary to take care ofa weakt-alker; simultaneously therewith decreases the receivingflpathimpedance if such change is necessary to prevent false operation of thetranstion of th applicants arrangements to the terminal of a four-wirecircuit.

The four-wire circuit includes a transmitting path L adapted fortransmission from a two-wire line L to a radio link or other mutablelink indicated at the right, and a receiving path L adapted fortransmission from the mutable link to the two-wire line L. Theconnection between the four-wire circuit and the line L is made throughthe usual arrangement of hybrid coil and artificial network whichsimulates somewhat roughly the impedance of the line L.

In order to prevent singing in the fourwire circuit the transmissionpath L is normally disabled at some point a. At the point I) in path L aportion of the outgoing energy is taken off through a transformer TRwhich has its primary winding m bridged across the line. The secondarywinding of this transformer is connected in the input circuit of anamplifier-deflector AD the output of which controls the singingsuppressor relay SS designed to remove the disability at point a and asecond relay ES designed to disable the path L at point 0 to preventechoes. If energy in the receiving path L is not blocked at point 0, aportion of the energy is taken off at point (Z through the transformerTR which has its primary winding x bridged across the line L Thesecondary winding g of the transformer TR is connected in the inputcircuit of the amplifier-detector AD the output of which controls therelay S. When this relay operates in response to the energy in path Lthe output circuit of amplifier-detector AD is broken and the operationof relays SS and ES in response to later arriving energy in the path Lis prevented.

Bridges across the transmitting path L is a three-electrode vacuum tubeT and a sec ond three-electrode vacuum tube T is bridged across thereceiving path L Each of these vacuum tube circuits, of course, provides a variable bridged impedance which, upon regulation of the voltageapplied to the grid, serves as a means for introducing a variable amountof loss in the associated transmission path.

The control of the volume in the transmitting path L is effected by theoperation of a potentiometer P which varies the amount of negativevoltage from battery B which is applied to the grid of tube T A secondpotentiometer P is operated simultaneously with the potentiometer P asindicated in the drawing, and serves to vary the amount of positivevoltage impressed on the grid of tube T from battery B. It will be notedthat as the potentiometer arms are turned simultaneously in a clockwisedirection, the negative voltage on the grid of tube T is increased whilethe positive voltage on the grid of tube T is increased. This operationof the potentiometers raises the bridged impedance of tube T and thuslowers the loss introduced in the transmitting path L (which changewould be required to take care of a Weak local talker) simultaneouslythis potentiometer operation, unless it is partially or fully of set,reduces the bridge impedance of the tube T and increases the lossintroduced in the receiving path L In order to prevent the lowering ofthe received volume by the decrease of the value of the bridgedimpedance through the tube T when such change is not necessary toprevent false operation of amplifier-detector AD the applicant providesfor the application of a variable negative voltage to the grid of tube Tas will now be more fully described. The switch CS may be termed theclipping control. When this switch is operated to position 1, 2 or 3, itcauses the operation of a relay S1 or a relay S or a relay S The voltageapplied to the first grid of the amplifierdetector AD is determined bythe portion of the secondary winding of transformer TR which isconnected into the input circuit of the first amplifier-detector tube,and it is the function of the chain of relays S S and S to control thissecondary winding connection. With the switch arm of CS on contact 1, asindicated in the drawing, relay S is operated and only a small portionof the winding 1 is connected into the amplifier-detector circuit. Thiscondition is one which gives a relatively low sensitivity toamplifier-detector AD and is suitable for a talker connected to line Lwho has high ability to operate the amplifier-detector and the relayscontrolled thereby. It will be understood that if such a talker haslower ability to operate this apparatus, the switch CS will be operatedto position 2 or position 3 to cause the operation of relay S or relay Sto render effective a greater portion of the winding y and thus increasethe sensitivity of the amplifier-detector AD Ability to operate relaysvaries with different talkers, even though the volumes representingtheir voices (usually measured with the use of strong sounds as acriterion) are regulated to give a constant value at point I).

It will be noted that when the switch CS is operated, circuit changesare made in the chain of relays R R and R associated with a battery BThis battery B is designed to impress a variable amount of negativevoltage on the grid of tube T or, more accurately, to contribute avariable amount of negative voltage to the circuit which controls thegrid voltage of the tube. With the arm of switch CS on contact 1, asindicated in the drawing, a relay R (in addition to relay S is operated,and the maximum value of negative voltage from battery B is applied.This comparatively large negative voltage impressed on the grid of tubeT tends to raise the bridged impedance. ,and: to incnease jormaintain'the volume in theiieceiving path Lg.

It willbe noted that as switch CS is operated 0 static; controlcorresponds; to the;l= switch CS described above; It operates. one relayof the chain' S S 1Aand1-S to control thej,

sensitivity of amplifierkdetector AD- lby 'ren ,v

dering effective the difi'erent 7 portions Qof the secondary winding 3of transformer TR which'is bridged across the receivingulpath L at point(1,1 Theoperationof swi'tchCS also controls the operation of the chainof circuit controlling thervoltage. impliesfi d on the grid oftube T avariable' amountiofne'gm ative'. voltageifrom'gbattery B. With;switch(ls jini position 1, as indicated, relays 'S i andfRf'are operated; Theoperation of' tivelyl'high static condition, Likewise the operation ofrelay R connects into the ,cir-

small'amountof negative voltage.

V The third switch shown CS; may be termed" the return loss control.and'serves merely'to connect into the circuit controlling thegrid B Thisvariation'is designed to take care 7 of diflerent return 'lo'ssesat thecircuit ter-i minal.

It is seen above of the operation of switches CS CS and CS that thesecontrols mutually contribute a negative'voltage from batteries B B5and-B to the grid of tube T and that the amount of negative voltagecontributed 345 by each of the switches dependsupon certain conditionsof the circuit's pecifically,'

the condition requiring adjustment of the sensitivity ofamplifier-detector AD the noise condition affecting the desirablesencircuit. It willbe'understood further that the algebraic sum of thenegative voltages from batteries B B and B and the positive voltage frombattery B, which is controlled by potentiometer P determines the i ingpath L or, in other words, the receiv-' ing volume willbe maintained atits normal level. If, however, the potentiometers P I and P are turnedso far in the clockwise direlays R1';,"R w1 andz R f to connectuintothecuit controlling the grid of {tube T -only a of tube 'l variousamountsof battery from' sitivity of amplifier-detector AD and the; return lossat the left end of the four-wire rection{ (to'take care of: a weak;talker-"con nected: to line L), that 1 the positive Ivoltage impressed,on y the I grid of tube T from bat.-

tery Bfis in effectfojnly partially cancelled out by ,the negativevoltage on the grid, the

the grid of tube T should be proportioned to produce this increase ofloss in the path Laponl-y when a failure to increase the loss -would;result;in a level in the transmitting path L high; enough 7 toncause theundesired operation of amplifieredetector AD andthe relays SS andEScontrolled thereby.

It will benoted that filters F and}? T are included in the gridcircuitsoftubes T and T respectively; "Iheseifilters. serve to. preventclicks inthe. respective transmission circuitsfdue" tofl the operation oftheflpoten tiomet'er vP the potentiometer. P er; one of the switchesCSgC Sgand CS J Three steps are shown for eachflofthe switches CS CS ,and CSand theivnlimb'er of relays corresponds. .It will be'understood thatthenumber of steps'and the correspondmgnumb ref relaysmay beincreased asre-"qulredl The specific disclosure made is merely for. the purpose ofillustration and adequately brings out the o"perations resulting j fromactuation' of any one of f the Q While 7' the invention has n disclosedin ab eta rmy o hep lrp s 'io lil uslitTwill bfu sto d t t' u hid r:6.10s ejis not neces'sarily'lirniting butftha't V r the' tr lscope ofthe invention determined from s ussions given in "oneidirectiom a secondpath adapted for transmission in the opposite directi'om-meansresponsive to energy in each of said paths for "control-ling jthetransmission over the paths,- a 'three electro'de vacuum tube bridgedacross lone of said paths for introducing a gal) variable loss therein,means forimpressing I avoltage onthe' grid of said tube,means forregulating the 'voltagefimpressed on said grid,and a lurality ofmutually contributing"means* 'or reflecting auxiliary regulationof thevoltage impressed on'said grid,

7 one of said plurality of means also acting toregulate' thesensitivityof each of said I V transmissioncontrolling means.

2; In a systemfor the-transmission o'f'energ'y, a first path adapted fortransmission infonefdirectiomga second path adaptedfor traiismission 'nthe opposite'j direction; means for normally disabling said first path,control means responsive to energy in said firstpath for removingthe'disabilitythere from? and for disabling said second path,

transmission path, and means associated with each of said sensitivityadjusting means for introducing auxiliary adjustment of said lossintroducing means in said second path.

3. In a system for thetransmission of en ergy, a transmitting path, areceiving path, means for normally disabling said transmitting path,control means responsive to energy in said transmitting path forremoving the disability therefrom and for disabling said receiving path,control means responsive to energy in said receiving path for preventingthe operation of said first-stated control means, means associated witheach of said control means for adjusting the sensitivity thereof, athree-electrode vacuum tube bridged across each of s'aidpaths forintroducing a variable loss therein, means for simultaneously eifectingcorresponding but opposite adjustments of the voltages on the grids ofsaid tubes, and a plurality of mutually contributing means for makingadjustment of the grid voltage of the tube bridged across the receivingpath opposite to that made simultaneously with the adjustment of thegrid voltage of the tube bridged across the transmitting path, one ofsaid mutually contributing means being operable simultaneously with eachof the means for adjusting the sensitivity of said control means.

4. In a system for the transmission of energy including a transmittingpath, a receiving path and means in each path for regulating the volumetherein, the method of transmission control which consists in makingsimultaneously an adjustment of the regulating means in the transmittingpath in one direction and a corresponding adjustment of the regulatingmeans in the receiving path in the opposite direction, and making aplurality of mutually contributing auxiliary adjustments of theregulating means in the receiving path which tend to oppose theadjustment made simultaneously with the regulation of the transmittedvol ume, whereby the regulation of the received volume will occur onlywhen the regulation of the transmitted volume exceeds a predetermineddegree.

5. In a system for the transmission of energy including a transmittingpath, a receivin'g path, means responsive to energy in each path forcontrolling the transmission over the system and means in each path forregulating the volume therein, the method of securing satisfactoryoperation of the transmission controlling means without unnecessaryreduction of the volume in the receiving path which consists in makingan adjustment of the volume regulating means in the receiving path whichtends to decrease the received volume simultaneously with an increase ofthe transmitted volume and in corresponding degree, adjusting thesensitivities of the respective transmission controlling means inaccordance with the transmission conditions in the respective paths, andmaking simultaneously with any sensitivity'adjustment an auxiliaryadjustment of the volume regulating means in the receiving path whichtends to oppose the adjustment thereof made simultaneously with thetransmitted volume regulation, whereby the received volume Will bedetermined by the algebraic sum of the principal adjustment and theseveral auxiliary adjustments of the regulating means in the receivingpath.

In testimony whereof, I have signed my name to this specification this17th day of September 1931.

DOREN MITCHELL.

A K i

